Shopify - Add a feature to collect details from the customer during purchase. Also add a feature for selecting a custom quantity.

I need to create two things for my website. 1. I need to collect specific details entered by a user. I sell digital goods so no physical delivery is required, but I do need to know specific information from the customer to be able to deliver their product. 2. I sell one product, but different quantities. Rather than having set products, with the option to buy 100,200,300 and so on, I would like to have it so customers can select a custom amount, for example 157, 243 and it works out the price and creates the item for them, working out the price on a ratio of what the cost of 1 is. I need to be able to adjust the price per 1, and also be able to set limits. It would be useful if I could also not allow a customer to buy more than the stock I have available, even if I have to enter the stock level somewhere else. More detailed information can be provided through messages.